Analyte concentration determination in biological fluids, e.g., blood or blood-derived products such as plasma, is of ever increasing importance to today's society. Such assays find use in a variety of applications and settings, including clinical laboratory testing, home testing, etc., where the results of such testing play a prominent role in the diagnosis and management of a variety of disease conditions. Common analytes of interest include glucose for diabetes management, cholesterol for monitoring cardiovascular conditions, and the like. In response to this growing importance of analyte concentration detection, a variety of analyte detection protocols and devices for both clinical and home use have been developed.
One type of method that is employed for analyte detection is an electrochemical-based method. In such methods, a sample of a substance to be tested, e.g., a biological substance typically in aqueous liquid form, e.g., blood, is placed into a reaction zone in an electrochemical cell made up of at least two electrodes, i.e., a counter/reference electrode and a working electrode. Typically, a redox reagent system is present within the reaction zone. Such a reagent system includes at least an enzyme(s) and a mediator. In many embodiments, the enzyme member(s) of the redox reagent system is an enzyme or plurality of enzymes that work in concert to specifically oxidize/reduce the analyte of interest. When the sample is deposited into the electrochemical cell, the targeted analyte comes into contact with the enzyme(s) and reacts therewith forming an oxidisable (or reducible) inactive enzyme. It is a mediator's role to react with an oxidisable (or reducible) enzyme generating a fully active enzyme and a substance, i.e., the product of the reaction between the inactive enzyme and the mediator, in an amount corresponding to the concentration of the targeted analyte. The quantity of the oxidisable (or reducible) substance present is then estimated electrochemically and correlated to the amount of analyte present in the initial biological substance.
The above-described electrochemical cell is commonly used in the form of a disposable test strip on which the biological sample is deposited and which is receivable within a meter by which the electrochemical analyte concentration is determined. Examples of assay systems that employ these types of test strips, often referred to as biosensors, and meters may be found in U.S. Pat. Nos. 5,942,102, 6,174,420 B1 and 6,179,979 B1, the disclosures of which are herein incorporated by reference. These systems can be characterized as coulometric, amperometric or potentiometric, depending on whether the system involves measuring charge, current or potential, respectively, in making the analyte concentration determination.
In electrochemical analyte measurement assays, it is necessary that the measurement system be able to detect the presence of a sample deposited onto a test strip so that the analyte concentration measurement test may be initiated. Moreover, it is important that the presence of sample be detected as soon as the sample comes into contact with the reagent system of the test strip. The timeliness of this detection is important in order to minimize the potential for perturbation of the electrochemical reaction between the target analyte and the reagent system. Perturbation is a change in the equilibrium of the electrochemical cell's reagent system caused by other than the normal and expected reaction progress of the target analyte with the reagent system mediator and enzyme components.
Perturbation is a particularly problematic with amperometric sample detection methodologies, known as “chronoamperometry,” which are employed in electrochemical analyte concentration determination methods, and most commonly employed in chronoamperometric assays of an analyte concentration. In many analyte concentration measurement methods, a constant-voltage step function is applied to the test strip, i.e., across the working and reference electrodes, which, upon sample application to the test strip, results in generation of a current through the electrochemical cell of the test strip. The magnitude of the applied voltage must be sufficient to trigger the Faradaic or capacitance current flow in the cell to provide rapid sample detection. When the current produced as a result of this applied voltage exceeds a predetermined threshold value, the system, i.e., the meter, “stamps” this time as the beginning of the analyte concentration measurement phase, and thus, initiates measurement of the current at the working electrode to determine the concentration of the targeted analyte. The electrochemical reaction between the redox reagent system and the biological sample is initiated prior to the system being ready to accurately stamp or mark the actual time of initiation of the analyte concentration measurement phase. As such, a fraction of the current produced as a result of this electrochemical reaction is used as part of the sample detection phase. Thus, the finally measured current is not an accurate representation of the analyte concentration of the sample. In other words, during the time prior to achieving the predetermined current value, i.e., prior to the sample detection time, the voltage applied to the cell will “perturb” the electrochemical reaction between the target analyte and the reagents. The shorter the device response time and/or the higher the applied voltage prior to achieving the requisite current threshold value, the greater the perturbation of an electrochemical reaction and, thus, the less accurate the analyte concentration measurement is likely to be.
Another disadvantage of the chronoamperometric method is that it is more likely to produce an inaccurate measurement with samples containing low concentrations of the targeted analyte or high concentrations of red blood cells or both. As the current produced upon application of voltage to the electrochemical cell generally decreases with decreasing analyte concentration (or with an increase in hematocrit levels), the longer the sample detection time the less likely the measured current will be an accurate representation of analyte concentration. On the other hand, setting a lower current threshold level will likely make the system more sensitive and falsely trigger by noise.
Another known sample detection method is disclosed in U.S. Pat. No. 6,193,873 B1, which is hereby incorporated by reference. This patent discloses a chronopotentiometric method which overcomes the problem of perturbation associated with chronoamperometric methods of sample detection. Instead of applying a constant-voltage step function, the chronopotentiometric method involves applying a small, constant-current step function to the test strip. The voltage across the working and reference electrodes is then monitored. Only when this voltage falls below a certain threshold voltage is the measurement of the analyte made by switching from application of a constant current to a constant voltage mode. Because a larger proportion of the resulting current measured following this point is representative of the analyte concentration, this method is far more accurate than the chronoamperometric method of sample detection determination.
In order to practice the method of the '873 patent, it is necessary to employ electronic circuitry which provides both a current source for the supply of the constant-current step function to the reagent test strip for performing the sample detection phase, as well as a voltage source for performing the analyte concentration measurement phase of the method. As such, the electronic circuitry further includes the necessary components to allow switching from the application of the current supply to application of the voltage supply at the precise time that the sample detection phase is complete.
Thus, it would be beneficial to provide an improved method of very accurately, expeditiously and immediately detecting the presence of a sample applied to an electrochemical test strip. Of particular benefit would be such a method and a system for implementing such method which do not require the application of a constant current or voltage step function for purposes of detecting the presence of a sample on a test strip. Preferably, such a system would require fewer electronic components than the one described in '873 patent.